Band separation system



Nov. 12, 1935. El. GREEN 2,020,409

' BAND SEPARATION SYSTEM Filed Aug. 15, 1933 2 Sheets-Sheet 2 ATTORNEY ggsz sg QR Patented Nov. 12, 1935 UNITED STATES PATENT OFFICE BAND SEPARATION SYSTEM Estill I. Green, East Orange, N. J., assignor to American Telephone and Telegraph Company, a corporation of New York Application August 15, 1933, Serial No. 685,273 I 20 "Claims.

electrical filter selection.

The usual method of selecting certain parts of a band of frequencies is to use electrical filters. If, for example, it is desired to divide a band of frequencies into two parts, a combination of high and low-pass filters may be used. In the present invention a method is described for dividing a band of frequencies which does not essentially involve the use of electrical filters. This method of dividing a band of frequencies is based upon the general principle of separately modulating differently phased components of the same carrier frequency with the band to be divided. The difier nt phase relations between the frequencies resulting from the two modulations are used as a basis for eliminating the band of frequencies either above or below the carrier frequency. The invention, therefore, provides a device to accomplish functions similar to those of wave filters and suited to a wide variety of applications.

The features of the invention will now be better understood from the detailed exposition which follows, when read in connection with the accompanying diagrams, Figures 1 to 5. Figure 1 shows an arrangement which might be used to divide a band of frequencies by phase discrimination. Fig. 2 illustrates in greater detail a possible embodiment of the arrangement shown in Fig. 1. Fig. 3 shows a phase divider for the carrier frequency to be used with third order modulation. Fig. 4 illustrates a third order modulating circuit. Fig. 5 indicates the relative amplitudes of the voltages over a band of frequencies which can be obtained by means of the phase shifting networks shown in Fig. 2, or in Fig. 7. Figs. 6 and '7 illustrate band phase shifting net- Works.

The essential principles of the invention will now be demonstrated in connection with Fig. 1. Consider a band of frequencies from some source of signals such as S6. This band may be represented as sin q t, sin q t sin q t, sin q t sin q,,t (1) the amplitudes of the frequencies composing the band do not affect the operation of the method, the amplitude indices have been omitted in expression (1).

The frequency intervals between successive components of the band may be of any width and may be equal or unequal. The band (1) may be located anywhere in the frequency spectrum.

However, in order to avoid difficulties in obtaining the desired lower side band in the output 5 of the modulators, it may be convenient to have the band (1) located so that (qn qs) 27F is less than 10 2 23 Zr and Zr is less than twice being the frequency at which division is desired and g and Zr There is generated in the carrier frequency oscillator CO the frequency i Zr qk qa qm Two components of this frequency differing in phase by electrical degrees are obtained in any desired manner by means of the phase divider PD. T ese components of the carrier frequency may be designated sin (1st and cos qst. Each of these components is then modulated with the frequency band (1) extending from sin out to sin qnt, in the modulators M131 and MDz, which may be of any suitable second order type. Only the lower sideband of the modulation process is retained in each case. The upper sideband may be suppressed by filters such as LPi and LPz of Fig. 1, or tuned circuits, or by phasing out in a single sideband modulator.

Assume, for the present, that second order modulators are used. As a result of the modula- 55 g and Zr such that tion of sin qst, the following band of frequencies is obtained in the output of modulator MD1:

while through modulation of cos qst, there is obtained from modulator MDz:

+cos (qt-11s) t Amplification is introduced by the amplifiers AM1 and AMz which have identical gain-frequency characteristics and introduce no phase distortion. These amplifiers supply the primaries of transformers 'IRi and TRz. Two of the output windings, one from each coil, are connected series aiding and the other two are connected series opposing. Thus the output into line L1 may be obtained by adding together the groups of frequencies represented by Equations (3) and (5) which will give the band of frequencies from The output into line L2 may be obtained by subtracting these groups since the windings of transformers IE1 and 'I'Rz oppose each other. This will give the band of frequencies from It will be apparent that these results will be obtained regardless of the phases or magnitudes of the frequency components of the original band extending from to 27F In the treatment just given the use of second order modulators was assumed. That the same result may be obtained using modulators of the third order type will now be demonstrated. Consider again the band of frequencies represented by Equation (1). Referring to Fig. 1 this band of frequencies is introduced to the modulators MDI and MDz in the same phase, these modulators now being of any desired type of third order device. As before it will be assumed that the band (1) is to be divided into two parts, the

point of division lying between the component frequencies The carrier frequency oscillator CO generates a frequency and 21 such that qk q s qm Two components of this frequency differing in phase by 45 electrical degrees are obtained from the phase divider PD. These components may be designated sin qst and sin Any odd multiple of 45 degrees [i. e.,

sin (qn2qs)t,

while modulation of sin f) (q 4/ will produce the following frequencies:

It will be noted now that the sign of the terms in Equation ('7) changes at the point where the division is to be accomplished. The procedure from this point on is the same as that considered under second order modulation. The phases of one of these sets of frequencies are shifted by 90 electrical degrees and the two groups are then added or subtracted to eliminate the undesired frequencies. The frequency components resulting from the addition of the frequencies represented by Equation ('7) to those of Equation (8) shifted 90 degrees in phase will now extend from qm" q s qn q s 21r 9 21r while subtraction will give frequencies from q e ql q sqk 21r 27 It is, therefore, plain that either second order or gi Zr and one of which is lower than and the other higher than the frequency at which it is desired to divide the band,

Let these frequencies have random phase angles, denoted by on and [3, respectively. Then the expression for these frequencies may be written Sin (q1 (qm B) where qk qs qnv Now modulate with these frequencies two component currents of the carrier frequency,

differing in phase by any angle 0, which may be represented as Assuming second order modulators, the lower sideband frequencies resulting from modulation of the first carrier component with (9) will be represented as follows:

and that from modulation of the second component COS l(qsqi=) l+ l(qm-qs) +B If now we shift the phases of the components of (13) by some angle we obtain Adding the lower sideband (12) to the other shifted by an angle (14) and combining we get:

sin q t, and sin (gi t-H9) The two terms of the above expression represent the two frequencies obtained by modulating the carrier frequency with the original signal frequencies and these frequencies, expressed by the last part of each, are

211' and Zr Of greatest interest, however, are the amplitude coeflicients of these two frequencies which are 2 cosQi-qi) and 2 cos%(0+ qi) In order to eliminate one of the sideband frequencies, one of these amplitude coefiicients must be reduced to zero, while the other remains. By inspection it will be obvious that this can readily be accomplished by making the sum or difference of 0 and equal 180 electrical degrees.

expressions (16) and (17) become 2 cos%(0)=2 cos (090)=2 sin a 19 2 cos%(0+)=2 cos =0 (20) or if 2 cos%(0-)=2 cos;l:90=0 (22) By satisfying Equation (18) or (21) it is obvious that either sideband frequency can be deleted, leaving the other. The coefficient of the remaining term, 2 sin 0, is of course, a maximum when 6:96", which is the special case which has been described previously. In this case is either +90 or 90, depending on which frequency it is desired to eliminate.

The frequencies used in this treatment,

are general expressions,

in and 2T being any frequency less than the carrier any frequency greater than the carrier. Thus the derivation may be applied to a whole band of frequencies, the relations true for g n and 2T being true for all the frequencies below the carrier, and those for holding for all the frequencies greater than the carrier. Thus, if the angle of phase shift is exactly the same angle at every frequency elimination of one of these bands of frequencies may be accomplished.

The expression (15) also indicates that the initial phase angles, a and 13 in this case, of the frequencies in the band to be divided are immaterial to the operation of the system, since they do not enter into the coefficient expressions (16) and (17).

The exposition of the general case has been made assuming the use of second order modulation. Third order modulation could be used, and the derivation would be similar. The angular phase difference between the. carrier compo:-

nents in this case is doubled in. the modulation products just as is the frequency of the carrier. The amplitude coefiicients, therefore, become 2 cos (26) 24) and 2 cos (20+) and the condition for annulling. one. of the frequency bands is It will now be of interest to consider further some of the details of the arrangement discussed in connection with Fig. 1 in which only Sill-degree phase shifts are employed. Referring to Fig. 2, a possible embodiment of the invention is shown. A source of signals SS is shown and transformers TR]. TRz are employed to introduce signals to both modulators M131 and MD2 in the same phase. A source of carrier frequency CO is shown. The output of this oscillator is impressed across a resistance R1 and condenser C1 in series. The voltage drop across the resistance R1 is displaced in phase by electrical degrees relative to that across condenser C1, the desired relation for second order modulation. The voltage drop across the resistance R1 is applied to the modulator MD1 by means of resistance R4 and that across C1 is introduced to MDz by means of resistance R5. The reactance of the condenser C1 at the carrier frequency is made equal to the resistance R1; that is,

where his equals 211- times the carrier frequency The components of the carrier frequency applied to the two modulators are, therefore, equal in magnitude and differ in phase by 90 degrees. The frequency of the carrier oscillator is that frequency at which it is desired to divide:

the incoming band of signals i g (21 to 271') The modulators are of the balanced type so that the carrier frequency is suppressed in the output.

These modulators are not necessarily of the vac uum tube type as shown but may be any type of second order modulator, such as copper oxide units. Only the lower sideband is desired and, therefore, the low pass filters LP1 and LPz are inserted after the modulators MDi and MDz, respectively. These filters do not need to have sharp shown in Fig. 2. The phase difference between the carrier voltage components should be an odd multiple of 45 electrical degrees, and the carrier frequency should be half that frequency at which it is. desired to. divide the frequency band 21 m (21! to 21) Considering the network shown in Fig. 3, the carrier frequency generated in the oscillator CO is which is half of the carrier frequency where ws equals 21r times the carrier frequency 3. 5. 21r

The voltage across this capacity-is then added to that across the portionof the resistance .414 R'1. Since these voltages are equal and in phase quadrature the resultant voltage E1 will bear a phase relation to the voltage across the remainder of the resistance which is an odd multiple of 45 degrees, inthis case 135 degrees The magnitude of this component will be proportional to or .586, which is the same as that across the remainder of the resistance (.586 R'l). The result of modulating the two components of the carrier frequency with the incoming band of frequencies will be equivalent to that obtained when second order modulators are used as described in connection with Fig. 2. The modulator in this case might consist of one or more units of thyrite (a mixture of finely divided particles of carbon and clay). A circuit arrangement for such a modulator, showing thyrite units TU1 and TUz replacing vacuum tubes such as VT1 and VTz of Fig. 2, is shown in Fig. 4.

Referring now to Fig. 2, the lower sideband from modulator MD1 is impressed on. the amplifier AM1 by means of transformer T135 and that from MDz on amplifier AMz by means of transformer TRE- Amplifiers AM1 and AMz'comprise vacuum tubes VTs and VT@ associated with transformers TIT-L5 and TRG and chokes CH1 and CH2. These elements should be substantially identical and the amplifiers are shown with common battery supplies in order that their gains may be kept equal.

resistance.

As previously explained, regardless of whether second or third order modulation is employed, the amplified output of one modulator should be shifted in phase degrees with respect to that I of the other. A possible method of shifting the ance R3. These elements have values It will be readily seen that these two networks PS1 and PS2 present equal impedances to the plate circuits of the vacuum tubes VTs and VTs. The resonant frequency of these networks is adjusted to some frequency within the frequency band occupied by the lower side-band of the modulation. The leads to the transformer TR7 are connected across a part of the resistance R2 and hence the voltage VB. is not shifted in phase with respect to the current.- These taps should be so located as to make the voltage Va equal to the output voltage VLC of the lower network. The voltage VLC will'be shifted 90 degrees out ofphase, however, as will now be explained, YObviously, the voltage drop across the condenser C2 of the upper network lags the voltage drop across R2 by 90 degrees at all frequencies,

i while the voltage acrossLz leads that across R2 .which for a considerable range on each side of -thc resonant frequency has substantially the same magnitude as the voltage drop Va across a part of R2 and difiers from it in phase by 90 degrees. This will be clear from the curve V1.0 of

Fig. 5 which shows the two reactive voltages combined with one voltage reversed. In the upper network the phases of the reactive voltages are of no importance as the output is taken from the In the lower network the output is taken from the reactances however, and the reactive voltages are added inreversed relation. This .addition of the voltages across the inductance and condenser is obtained by the use of the parallel circuit, as indicated in network PS2. The reversal of the one reactive voltage with respect to the" other is brought about by reversing the order of the inductances and capacities in the parallel branches, the taps being taken at the junction points of the reactances in the two branches. The voltage VLC is thus obtained .by superposing the voltage drops across the inductances L3 upon the voltage drops across the condensers C3 with their signs reversed. As will be shown below by a concrete example, this voltage V1.0 may be made substantially equal, over a considerable range of frequencies on either side of the resonant frequency of the L2C2 circuit, to the voltage VR. which is the drop across a part of the resistance R2 of the upper network PS1.

In order to obtain a concrete example of the operation of the phase shifters, PS1 and PS2 in as shown by the curve V1.0 in Fig. 5. voltage V12 this case is selected as .22 times I reactive voltage drops.

Fig. 2, let it be assumed that the resonant frequency 1/ 2 2 V is 1000 cycles, that the value of R2 is 1000 ohms and that L20: and

each have a value of ohms at 1000 cycles. Let us assume for the moment that the same voltage E is applied to both the upper and the lower resonant circuits. If reactive voltages are reversed with respect to each other the magnitude of the combined voltages over the range of frequencies from 500 to 2000 cycles will be The the voltage across R2, because that is about the quency range plotted.

It is evident, therefore, 'in connection with Fig. 2, that by the use of the two networks PS1 and PS2 we can shift one of the applied bands of frequencies exactly 90 degrees in phase relative to the other while producing only a small magnitude distortion over a fairly wide frequency band. The lack of'precise equality of magnitude results in a small amount of crosstalk or interference from the unwanted band into the output of the wanted band. It should be noted in connection with the method used to shift the phases of one of the sidebands resulting from the modulation, that the networks PS1 and PS2 in Fig. 2 accomplish the desired result only for a limited frequency band. This frequency band is limited by the magnitude relations between the resistive and the combined Too large a diiference in the magnitudes of these voltages would impair the performance of the system. From Fig. 5 it will be seen that in the bandfrom about 500 cycles to 2000 cycles the resistive and reactive voltages are substantially equal for the circuit values chosen above. By the use of more complicated networks or other phase shifting devices it would be possible to obtain more precise results with respect to the shifting of the phase of one frequency band relative to the other, and to extend the range of frequencies over which satisfactory results may be obtained.

Returning now to Fig. 2, these voltages VR and V1.0 are introduced to the amplifiers AM; and AM; by means of transformers TH; and 'IRa. These amplifiers should have identical gainfrequency characteristics. After being amplified, the side-bands are combined at the outputs of transformers TRQ and 'I'Rm by the switch SW. Addition of the side-bands will produce the sideband frequencies corresponding to that part of the original band Q] In 27r to 21r which lies above the carrier frequency, that is, the band qm' qa qn qa. 21x 21r By reversing the switch SW, the difference of the two sidebands will give the sideband frequencies corresponding to that part of the original band below the carrier frequency, or

In the systems illustrated-in Figs. 2, 3 and 4, the carrier components supplied to the two modulators are shifted relatively either 90 degrees or 45 degrees, and the relative phase shift introduced by the networks PS1 and PS2 is also 90 degrees. If phase shifts other than these are to be used in accordance with the principles outlined in connection with Equations (9) to (26) inclusive, some modification in the networks employed will be necessary.

Any desired relative phase shift betweenthe carrier components supplied to the two modulators may be obtained by using a carrier supply circuit of the type shown in Fig. 3. The particular distribution of resistance between the two halves of the circuit shown in Fig. 3 produces a relative phase shift of 45 degrees. By merely changing the ratio of the two parts of the resistance in the two halves of the circuit, relative phase shifts either greater or smaller than 45 degrees will result. For example, by making the resistance in the upper section smaller and smaller, with corresponding increase of that in the lower sections, the phase shift will be increased until in the limiting case, with zero resistance above and full resistance Rr below, the phase shift will be degrees. On the other hand, if the upper resistance is increased and the lower resistance is decreased the phase shift may be made to approach zero.

To obtain a relative phase shift other than 90 degrees in the selected output bands of the two modulators, the lower network PS2 may be replaced by the network shown in Fig. 6, keeping the simple series networkPSi in the upper branch the same as in Fig. 2. In Fig. 6 it will be observed that a portion (aR) of the total series resistance R is removed from the common branch of the network, thus leaving the remainder (la)R in the common branch, the removed part being in effect divided between the two parallel branches by including a resistance element having a value 2aR. in each parallel branch. The three impedance elements in each branch are arranged so their order is reversed in one branch.

By comparing this network with the network PS2 of Fig. 2 in which the reactance elements introduce a phase shift of 90 degrees between the output voltage V1.0 and the voltage across the resistance Ra, it will be evident that the effect of the resistance elements in the two parallel branches in Fig. 6 is to produce a phase shift of less than 90 degrees. This will be clear when we consider that in network PS2 the drop across the inductance 2L2 in one of the parallel branches is 90 out of phase with the drop through the series resistance R3, while in Fig. 6 the drop through the resistance 211R in series with inductance 2L in one branch will be out of phase with the drop through resistance (1-a)R. by an amount determined by the vectorial sum of the drops through resistance 2aR and inductance 2L.

From what has already been stated with reference to the action of the networks PS1 and PS2 of Fig, 2, in which two separate voltages applied to the networks may be relatively shifted in phase 90 degrees, it will be evident thatby an analogous arrangement an applied voltage may be translated into two separate voltages in phase quadrature. Such an arrangement is shown in Fig. 7. Here the applied voltage V0 is applied to a network somewhat like network PS2 of Fig. 2, said network having a resistance R in series with two parallel branches each having of taps which is in phase quadrature with re 5 spect to a voltage V1.0 taken off the upper set of taps, By properly setting the taps these two voltages may be made substantially equal in magnitude over a considerable range of frequencies.

Having obtained by any of the methods previ- 1Q ouslydescribed, separate frequency bands representing the two desired. portions of the original ban-d of frequencies, modulated to the carrier frequency, it is possible to translate either band to any desired position in the frequency spectrum, 15 including, of course, that originally occupied by said band. A single sideband modulator of any of several well known types may be used for this purpose, such as,'for instance, those disclosed in my patents referred to above. 20

By additional steps of division in the manner already described, the original frequency band may be divided into any desired number of separate bands of frequencies. In this way my invention may be made to perform the functions of 25 one or more band filters, as well as those of high and low pass filters.

As compared with selection by the use of filters, where the dead space or separations between adjacent selected bands becomes greater as we go up so in the frequency spectrum, the method of selection herein disclosed has the advantage that the separation between bands is the same at higher frequencies as at low frequencies. Hence even if the design is such that the cutoff is not as sharp 35 as a band filter at low frequencies, it will give sharper selection at high frequencies.

It is scarcely necessary to discuss the applications of my invention. It has been disclosed as a method and means for performing functions simi- 40 lar to those of high and low pass filters. With additional steps the functions of band selection and band elimination filters may also be performed. Itis obvious that a device capable of performing the above functions will have manl- 5 fold uses. .One possible application, will however, be mentioned by way of illustration.

A possible application of this method of dividing a band of frequencies is in the reception of a single sideband of a double sideband transmission. 50 The arrangement of apparatus shown in Fig. 2 may be easily adapted for radio reception. The source of signals SS will for this purpose comprise a radio antenna with or without a radio frequency amplifier and tuned circuits for discriminating in 55 favor of the desired radio transmission. Both sidebands will be transmitted to the modulators MD1 and MDz. The carrier frequency oscillator CO will generate a frequency equal to the carrier frequency of the radio transmission, or it might 30 even be derived from the radio carrier. The operation of the apparatus has already been described and it will be seen that by either adding or subtracting the two demodulated waves, at the output, the resulting audible frequencies obtained 65 will correspond to either the upper or the lower sideband of the original transmission.

While this invention has been disclosed as em bodied in certain particular-forms, it is capable of embodiment in other and different forms without 70 departing from the spirit and scope of the appended claims. 3

What is claimed is:

l. The method of dividing a band of frequencies, which consists in producing differently 7 phased components of the same carrier frequency, modulating one of said components with a part of the energy of said band, modulating the other component with an identically phased part of the energy of said band, and utilizing the different phase relations between frequencies resulting from the two modulations as a basis for obtaining two separate bands of frequencies which correspond, respectively, to different sub-bands of said original frequency band.

2. The method of dividing a band of frequencies, which consists in separately modulating differently phased components of the same carrier frequency with said band, after such modulation shifting the phase of certain component frequencies resulting from one modulation with respect to corresponding component frequencies of the other modulation, and combining said phase shifted components with said other components so as to obtain frequency bands corresponding to certain portions only of the original band.

3. The method of dividing. a band of frequencies, which consists in separately modulating differentiy phased components of the same carrier frequency with said band, after such modulation shifting the phase of certain component frequencies resulting from one modulation with respect to corresponding component frequencies of the other modulation, and combining said phaseshifted components with said other components so as to obtain a frequency band corresponding to a portion only of the original band and suppress another portion of the original band.

4. The method of dividing a band of frequencies, which consists in producing two carrier currents of the same frequency and having a predeterminedphase relation, said carrier frequency bearing a predetermined relation to the frequency at which it isqdesired to divide said band of frequencies, separately modulating each of said carrier currents with sai-d band of frequencies, re-

taining only the lower sideband of each modulan, shifting the phases of the frequencies of bands so that portion of the original band are obtained in one to d amount with respect to those of the other of said bands, and combining the two resultant bands so that frequencies corresponding to one portion of the original band are obtained.

5. The method of dividing a band of frequencies, which consists in producing two carrier currents of the same frequency and having a predetermined phase relation, said carrier frequency hearing a predetermined relation to the frequency at whi h it is desired to divide said band of freq'uen ies, separately modulating each of said carrier currents withsaid band of frequencies, reto ng only the lower sideband of each modulation, shifting the phases of the frequencies of one of said lower sidebands uniformly by a predeteramount with respect to those of the other of said bands, and combining the two resultant frequencies corresponding to one each modulation, shifting the phases of the frequencies of one of said lower sidebands uniformly by a predetermined amount with respect to those of the other of said sidebands, and combining the resultant bands so that a frequency band cor- 5 responding to the desired portion of the original band is obtained and components corresponding to the unwanted portion of the original band are substantially annulled.

7. The method of separating out a component 10 band of frequencies from a larger band, which consists in dividing said larger band by the method of claim 6, and dividing again by the same method the band resulting from said firstdivision.

8. The method of segregating a portion of a frequency band, consisting in generating .a carrier frequency at which it is desired to divide said band, obtaining two components of said frequency equal in magnitude but differing in phase by some angle, modulating each of said carrier frequency currents separately with said frequency band by second order modulation, retaining only the lower sidebands resulting from said modulations, shifting the phases of the frequencies of one of said 5 sidebands by some second angle relative to those of the other of said sidebands. combining said sidebands, and ohoosingsaid first mentioned and said second mentioned angles of phase shift so that t sum is equal to 180 electrical degrees,

enminating by said combination the frequencies on one side of the carrier frequency.

9. The method of segregating a portion of a frequency band, consisting in generating a carrier frequenc" at which it is desired to divide said hand, obtaining two components of said frequency equal in magnitude but differing in phase by some angle, modulating each of said carrier frequency currents separately with said frequency band by second order modulation, retaining only the lowor sidebands resulting from said modulations, producing a phase shift at all frequencies of one of said sideb-ands of some second angle relative to those of the other sideband, combining said sidebands, and choosing said first mentioned and 'said second mentioned angles of phase shift so that their difference is equal to 180 electrical degrees, thus eliminating by said combination the frequencies on one side of the carrier frequency.

1D. The method of div'ding a band of frequencies, which consistsin poducing two carrier currents of the same frequency having a quadrature phase relation, said carrier frequency lying between adjacent component frequencies of said band of frequencies, separately modulating by second order modulation each of said carrier cmrents with said. band of frequencies, retaining only the lower sidebands resulting from said modulations, shifting the phases of the currents of one of said lower sidebands 9 electrical de rees with respect to the corresponding currents of the other of said lower sidebands, and combining said phase-shifted sideband with said other sideband in two circuits so that the sideband frequencies bonding to one portion of the original band .a 'in one of said circuits, and those corresponding to the other portion appear in the other of said circuits.

11. The method of segregating a portion of a band of frequencies, consisting in generating a carrier frequency corresponding to one boundary of said portion, 0- ta ning two equal current components'cf said car for frequency in phase quadrature, modulating by second order modulation each of said components separately with said band of frequencies, retaining only the lower sidebands resulting from said modulations, producing a phase shift of 90 electrical degrees in one of said lower sidebands, and combining said phase-shifter sideband with the other of said sidebands in such a way that the frequency band corresponding to that portion of the original band to be segregated is retained, the undesired frequency band being annulled. 12.'The method of separating out a component band of frequencies from a larger band, consisting in segregrating a portion of said larger band by the method of claim 11, and by the same method segregating a portion of the resulting band.

l3.-The method of dividing a band of frequencies, consisting in producing two carrier currents of the same frequency having a phase difference equal to electrical degrees multiplied by an odd number, said carrier frequency having a value of one half the frequency at which it is desired to divide said band of frequencies, separately modulating by third order modulation each of said carrier currents with said band of frequencies, retaining only the lower sidebands of said modulations, shifting the phases of the currents of one of said lower sidebands 90 electrical degrees relatively to the corresponding currents of the other of said lower sidebands, and combining said phase-shifted sideband with said other sideband so that the sideband frequencies corre sponding to one portion of the original band are obtained.

14. The method of dividing a band of frequencies, consisting in producing two carrier currents of the same frequency having a phase difference equal to 45 electrical degrees multiplied by an odd number, said carrier frequency hav ing a value of one half the frequency at which it is desired to divide said band of frequencies, separately modulating by third order modulation each of said-carrier currents with said band of frequencies, retaining only the lower sidebands of said modulations, shifting the phase of the currents of one of said lower sidebands 90 electrical degrees relatively to the corresponding currents of the other of said lower sidebands, and combining said phase-shifted sideband with said other sideband so that the sideband frequencies corresponding to one portion of the original band are obtained in one circuit, and those corresponding to the other portion are obtained in another circuit.

15. In a system for dividing a band of frequencies, a source of signals, a pair of modulators of the second order type, means for introducelectrical degrees relative to those of the lower sideband from the other of said modulators, means for combining said phase-shifted sideband with said other means so that the resultant sideband frequencies correspond to the frequencies in the original band of signals on one side of said carrier frequency.

16. In a system for dividing a band of frequencies, a source of signals, a pair of modulators of the second order type, means for introducing signals from said source into said modulators in the same phase, a source of carrier frequency, said carrier frequency lying between adjacent component frequencies of said band of frequencies, means for introducing into said modulators currents of said carrier frequency equal in magnitude but in phase quadrature, means for shifting the phases of the frequencies of the lower sideband from one of said modulators 90 electrical degrees relative to those of the lower sideband from the other of said modulators, means for combining said phase-shifted sideband with said other sideband so that the resultant sideband frequencies correspond to the frequencies in the original band of signals on one side of said'carrier frequency, means for combining said phase-shifted sideband with the other sideband so that the resultant sideband frequencies correspond to those frequencies in the original band on the other side of said carrier frequency.

17. In a system for dividing a band of frequencies, a source of signals, a pair of third order modulators, means for introducing signals from said source into said modulators in the same phase, an oscillator generating a carrier fre quency having a value of half the frequency at which it is desired to divide said band of frequencies, means for introducing into said modulators currents of said carrier frequency equal in magnitude but differing in phase by 45 electrical degrees multiplied by an odd number, means for shifting the phases of the frequencies of the lower sideband from one of said modulators 90 electrical degrees relative to those of the lower sideband from the other of said modulators,

means for combining said phase-shifted sideband with said other sideband so that the resultant sideband frequencies correspond to the fre quencies in the original band of signals on one side of said carrier frequency.

18. In a system for dividing a band of frequencies, a source of signals, a pair of third order modulators, means for introducing signals from said source into said modulators in the same phase, an oscillator generating a carrier frequency having'a value of half the frequency at which it is desired to divide said band of frequencies, means for introducing into said modulators currents of said carrier frequency equal in magnitude but differing in phase by 45 electrical degrees multiplied by an odd number, means for shifting the phases of the frequencies of the lower sideband from one of said modulators 90 electrical degrees relative to those of the lower sideband from the other of said modulators, means for combining said phase-shifted sideband with said other sideband so that the resultant sideband frequencies correspond to the frequencies in the original band of signals on one side of said carrier frequency, and means for combining said phase-shifted sideband with the other sideband so that the resultant sideband frequencies correspond to those frequencies in the original band on the other side of said carrier.

19. In combination, a source of frequencies extending over a range, means for deriving from said source two components one of said components having a phase shift'at .each frequency of approximately 90 electrical degrees relative to the corresponding frequency of the other of said components, the relative current magnitude at each frequency in said two components being substantially constant, said means consisting in an impedance network comprising two identical series circuits of inductance and capacitance connected in parallel, one reversed with respect to the other, and a. resistance in series with said parallel combination, one of said components being the voltage between the midpoints of said two series circuits, and the other the voltage across part of said resistance.

20. The method of receiving a radio transmission comprising two sidebands and a carrier frequency, consisting in producing locally a carrier frequency substantially equal to that of said radio transmission, obtaining from said local carrier frequency two currents having a quadrature phase relation, demodulating in second order demodulators said radio transmission with each of said locally produced carrier currents, producing a 90-degree phase shift in one of the demodulated frequency bands after demodulation, and combining the demodulated frequency hands after such shift in phase in such a way that only the frequencies resulting from the demodulation of one of said side-bands are retained, the frequencies resulting from the other sideband being annulled.

ES'I'ILL I. GREEN. 

